The present invention relates to a position locating device for use in radiotherapy, and more particularly to a device for locating on the surface of a body portion the points of entry and exit of hollow stainless steel needles for interstitial implantation of radioactive materials.
The interstitial implantation of radioactive isotopes is a well established technique for the treatment of malignant tumors. Short lengths of a radioactive source, such as Iridium-192, are linearly arranged within thin walled plastic tubes to form "ribbons." These ribbons are used as temporary interstitial implants in a wide variety of clinical situations. Each ribbon, which is very flexible, must be inserted into the body with the aid of a rigid trocar, in the form of a hollow stainless steel needle, which is sharpened at one end for piercing the skin and tumor bearing tissue. An array of the hollow needles are first inserted into the tissues to be implanted. The needles are typically spaced evenly throughout the volume of tissue to be irradiated, and are parallel to one another to avoid areas of underdose or overdose. After the hollow needles have been inserted, ribbons of radioactive sources are inserted into the bores of the needles to positions determined by the location of the tumor bearing tissue. In certain clinical situations the needles may be left in place in the body, along with the sources, for the duration of the implant, after which both are removed. In other situations, the needles may be pulled out of the body, leaving behind only the ribbon sources for the duration of the implant.
The clinical applications of the present invention relate to the implantation of such needles, which may also be pushed through the tissue and thereafter removed, leaving behind the ribbons of sources for the duration of the implant. The ribbons are typically anchored in place by crimping each of their free ends with a small metal disk or "button." When the desired radiation dose has been delivered, each ribbon is removed from the body by cutting off the button at one end and pulling the ribbon, by its opposite end, out through the skin. The classic technique for implantation of radioactive materials through soft tissue is illustrated and described on page 89 in connection with FIGS. 1-72 of the "Textbook of Radiotherapy," Third Edition, 1980, published by Lea & Febiger, Philadelphia, Pennsylvania.
In interstitial radiotheraphy, precision in the placement of the radioactive sources is important to the success of the treatment. Because of the limited volumes of the tissues being irradiated, and the rapid change in the intensity of radiation over small increments of distance from the sources, imprecise placement of the radioactive sources can result in areas which are subjected to underdoses and areas which are subjected to overdoses of radiation. Interstitial implants in the peripheral soft tissues of the body, such as in the extremities, are commonly arranged to provide one or more planes of radioactive sources. Each plane consists of multiple ribbons of sources which should be equidistant from one another and parallel. The key to obtaining planes of equally spaced and parallel ribbons of sources is to accurately locate the positions of the entry and exit points for the needles on the surface of the skin. This will determine the ultimate positions of the sources within the body. In the prior art, this position locating was performed by a free-hand method, using a ruler to mark out a row of equally spaced entry points on the skin on one side of the body. A row of exit points were then similarly marked on the opposite side of the body, while using visual sighting to try to keep the points of both rows in register in order to form a plane at the desired depth within the body. At best, the free-hand method is only an approximation, and can result in uneven spacing and non parallelism of the ribbons of sources and the planes formed by them. It is therefore desirable to provide an alternative to the free-hand method of locating the positions of the entry and exit points, especially on a body surface of non-uniform curvature, which may require modification in the placement of the entry and exit points to take into account continuous variations in the curvature of the body portion being implanted.